It is said that development of Kṛṣṇa consciousness is exhibited by proportionate material detachment, or vairāgya. If one does not separate himself from material enjoyment, it is to be understood that he is not advancing in Kṛṣṇa consciousness. Renunciation in Kṛṣṇa consciousness is so strong that it cannot be deviated by any attractive illusion. One has to perform devotional service in full tapasya, austerity. One should fast on the two Ekādaśī days, which fall on the eleventh day of the waxing and waning moon, and on the birthdays of Lord Kṛṣṇa, Lord Rāma and Caitanya Mahāprabhu. There are many such fasting days.
(Srimad Bhagavatam-----3:27:22-----purport).

A devotee is not concerned with enjoyment or distress; he simply desires to execute devotional service. It is said in the Śrīmad-Bhāgavatam that devotional service should be apratihatā, unchecked by the material conditions of happiness or distress. The devotee undergoes processes of austerity such as observing Ekādaśī and similar other fasting days and refraining from illicit sex life, intoxication, gambling and meat-eating. Thus he becomes purified from the reactions of his past impious life, and because he engages in devotional service, which is the most pious activity, he enjoys life without separate endeavour.
(Srimad Bhagavatam-----4:12:13-----purport).

We can definitely see that to advance in Kṛṣṇa consciousness one must control his bodily weight. If one becomes too fat, it is to be assumed that he is not advancing spiritually. Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura severely criticized his fat disciples. The idea is that one who intends to advance in Kṛṣṇa consciousness must not eat very much. Devotees used to go to forests, high hills or mountains on pilgrimages, but such severe austerities are not possible in these days. One should instead eat only prasāda and no more than required. According to the Vaiṣṇava calendar, there are many fasts, such as Ekādaśī and the appearance and disappearance days of God and His devotees. All of these are meant to decrease the fat within the body so that one will not sleep more than desired and will not become inactive and lazy. Overindulgence in food will cause a man to sleep more than required. This human form of life is meant for austerity, and austerity means controlling sex, food intake, etc. In this way time can be saved for spiritual activity, and one can purify himself both externally and internally. Thus both body and mind can be cleansed.
(Srimad Bhagavatam-----4:28:35-36-----purport).

In the Brahma-vaivarta Purāṇa it is said that one who observes fasting on Ekādaśī day is freed from all kinds of reactions to sinful activities and advances in pious life. The basic principle is not just to fast, but to increase one's faith and love for Govinda, or Kṛṣṇa. The real reason for observing fasting on Ekādaśī is to minimize the demands of the body and to engage our time in the service of the Lord by chanting or performing similar service. The best thing to do on fasting days is to remember the pastimes of Govinda and to hear His holy name constantly.
(Nectar of Devotion).

Those who suffer from diseases of the stomach must be unable to control the urges of the belly, at least according to this analysis. When we desire to eat more than necessary we automatically create many inconveniences in life. However, if we observe fasting days like Ekādaśī and Janmāṣṭamī, we can restrain the demands of the belly.
(Nectar of Instruction).